gdb: pass frames as const frame_info_ptr &

We currently pass frames to function by value, as `frame_info_ptr`.
This is somewhat expensive:

 - the size of `frame_info_ptr` is 64 bytes, which is a bit big to pass
   by value
 - the constructors and destructor link/unlink the object in the global
   `frame_info_ptr::frame_list` list.  This is an `intrusive_list`, so
   it's not so bad: it's just assigning a few points, there's no memory
   allocation as if it was `std::list`, but still it's useless to do
   that over and over.

As suggested by Tom Tromey, change many function signatures to accept
`const frame_info_ptr &` instead of `frame_info_ptr`.

Some functions reassign their `frame_info_ptr` parameter, like:

  void
  the_func (frame_info_ptr frame)
  {
    for (; frame != nullptr; frame = get_prev_frame (frame))
      {
        ...
      }
  }

I wondered what to do about them, do I leave them as-is or change them
(and need to introduce a separate local variable that can be
re-assigned).  I opted for the later for consistency.  It might not be
clear why some functions take `const frame_info_ptr &` while others take
`frame_info_ptr`.  Also, if a function took a `frame_info_ptr` because
it did re-assign its parameter, I doubt that we would think to change it
to `const frame_info_ptr &` should the implementation change such that
it doesn't need to take `frame_info_ptr` anymore.  It seems better to
have a simple rule and apply it everywhere.

Change-Id: I59d10addef687d157f82ccf4d54f5dde9a963fd0
Approved-By: Andrew Burgess <aburgess@redhat.com>

gdb/amd64-netbsd-tdep.c

@@ -35,7 +35,7 @@
    routine.  */
 
 static int
-amd64nbsd_sigtramp_p (frame_info_ptr this_frame)
+amd64nbsd_sigtramp_p (const frame_info_ptr &this_frame)
 {
   CORE_ADDR pc = get_frame_pc (this_frame);
   const char *name;
@@ -48,7 +48,7 @@ amd64nbsd_sigtramp_p (frame_info_ptr this_frame)
    return the address of the associated mcontext structure.  */
 
 static CORE_ADDR
-amd64nbsd_mcontext_addr (frame_info_ptr this_frame)
+amd64nbsd_mcontext_addr (const frame_info_ptr &this_frame)
 {
   CORE_ADDR addr;